The present invention relates to measurement or control of the distance from a focus point of a laser beam to a material surface of a workpiece to be machined in laser cutting and drilling systems. For simplicity, the term xe2x80x9cmachiningxe2x80x9d (and other variations such as xe2x80x9cmachinedxe2x80x9d) is used herein with respect to the present invention to refer to and is intended to include cutting, drilling, welding and similar operations performed by laser on metal and other materials. Such machining systems require the focus point of the laser beam to be accurately and repeatably located at the surface, or at a predetermined distance from the surface, of a part to be machined. Prior art systems typically used a sensor to detect the distance from the nozzle tip to the part surface. A common type of sensor used in the prior art was based on measuring the capacitance between the nozzle and the part surface. Such capacitance sensors were used to gauge the relative location of the laser beam focal point with respect to the part surface. The nozzle was then moved toward or away from the part surface to maintain a preset capacitance, which in turn was to maintain the laser beam focus point location. This prior art method was found to be satisfactory for part surfaces that are electrically conductive and generally normal to the direction of the laser beam. However, such prior art capacitive sensors were unable to satisfactorily perform with non-conductive part surfaces.
Capacitive sensors also exhibited an undesirable characteristic known as xe2x80x9cside sensingxe2x80x9d which occurs when the side of the sensing nozzle approaches the part surface or part fixturing. The relative movement of the side of the nozzle contributes to the capacitance change seen by the sensor and an accurate distance between the nozzle tip and part surface to be machined or drilled is no longer maintained. Such undesirable side sensing can occur as a result of part surface geometry, particularly where features that are to be laser processed are located near surfaces that are perpendicular thereto, or nearly perpendicular thereto. Undesirable side sensing influences on capacitance-based measuring systems will result whenever a part surface to be laser processed has a nearby contour projecting out of a plane containing the part surface and other than perpendicular to the laser beam emitted from the nozzle tip towards the part surface.
An example application where the limitations of capacitive sensing are significant is in the drilling of shallow angle holes in turbine engine components which may be formed of or coated with non-conductive material. In such an application, it has been found necessary to position the nozzle at a shallow angle, often as little as 15 degrees with respect to a plane tangent to the part surface at the location to be drilled. At shallow angles such as this, side sensing causes inaccuracies in the measured distance between the nozzle and the part surface. Even if the system is calibrated to compensate for side sensing errors, sensing at shallow angles results in the system sensing part deviations at a location away from the processing point. Because of this, the system may react to a change in material dimension that is not present at the processing point, but is present within the sensing region. Conversely, the system may not react to a change in material dimension at the processing point, when that change is masked or contra-indicated by a change in material dimension away from the processing point. At shallow angles, inaccuracy in the distance measured will not only affect hole quality, due to the focal point being improperly positioned, but will also induce hole location errors due to Abbe error induced at the shallow angle. Although there are methods such as the one described in U.S. Pat. No. 5,340,962 to prevent Abbe error, if the surface is not electrically conductive, the capacitive sensing technique will not accurately sense the actual part surface. Other techniques, such as contact sensors and eddy current sensors are subject to various shortcomings in the application of laser machining and hole drilling of interest here. Contact sensors are not effective because the debris generated by the laser process has been known to accumulate between the sensor and part surface to be sensed, causing erroneous readings. Heat and plasma generated by the laser process have been known to damage eddy current sensors, which must be located at the nozzle tip.
The present invention utilizes a sensor based on the confocal holography principle, and uses equipment available from the Optimet division of Ophir of Jerusalem, Israel. Linear conoscopic holography is explained in U.S. Pat. No. 5,953,137.
Furthermore, unlike certain prior art systems which have an adjustment mode followed by a rearrangement of parts to place the system in an operating mode, the sensor and measurement beam of the present invention is permanently installed and remains operational during machining operations in the practice of the present invention, notwithstanding that the focusing element may be changed, depending upon the application. The present invention also has advantages over the prior art in that the present invention has the ability to track changes in the focal point of the machining laser after changing the focusing element, because of the common optical path including the focusing element, shared by the machining laser and measurement beam. Additionally, because of the shared optical path, including the focusing element, the present invention automatically compensates for thermal effects on the focusing element, an effect known as xe2x80x9cthermal lensingxe2x80x9d when the focusing element is a lens.